3,4-Dihydrocoumarin is obtained by cyclization and dehydrogenation of a 3-(2-cyclohexanoyl)propionic acid ester in the presence of a solid metallic catalyst, etc. Coumarin by-produced by the reaction is separated from the reaction mixture by distillation, crystallization or the like means for use as a product. The by-produced coumarin may also be converted to 3,4-dihydrocoumarin by partial hydrogenation.
Other by-products resulting from the above reaction mainly include compounds having a lower boiling point than 3,4-dihydrocoumarin, e.g., ethylbenzene, o-ethylphenol, and a dihydrocinnamic acid ester; compounds having a higher boiling point than 3,4-dihydrocoumarin, such as esters (e.g., o-ethylphenyl 3-cyclohexylpropionate and o-ethylphenyl dihydrocinnamate), octahydrocoumarin, and dihydrocinnamic acid; and tar-like substances. In order to remove these by-products, the reaction mixture per se has conventionally been subjected to rectification to recover 3,4-dihydrocoumarin.
However, the conventional purification process being followed, o-ethylphenol and dihydrocinnamic acid are incorporated into the 3,4-dihydrocoumarin fraction during rectification to reduce the purity of the resulting 3,4-dihydrocoumarin.
Because o-ethylphenol has a low boiling point and gives off a phenol-like smell, it deteriorates the odor of 3,4-dihydrocoumarin for use as a perfume. Therefore, there is a demand to reduce the o-ethylphenol content in the resulting 3,4-dihydrocoumarin to about 0.05% by weight or less.
Similarly, dihydrocinnamic acid has a cinnamon-like smell and deteriorates the odor of 3,4-dihydrocoumarin for use as a perfume. Therefore, there is a demand to reduce the dihydrocinnamic acid content in 3,4-dihydrocoumarin to about 0.05% by weight or less.
In order to recover high purity 3,4-dihydrocoumarin free from impurities, complicated steps such as repetition of rectification have been required, and it also leads to a loss of 3,4-dihydrocoumarin.